The design principles of diffractive optics date back to microwave devices, where an array of metallic antennas was used to control the radiation pattern of an incident electromagnetic wave. With progress in fabrication techniques, numerous planar devices, such as gratings, lenses, polarization-sensitive devices, and mirrors have been realized in the infrared and visible spectrum. Metasurfaces have added another dimension to diffractive optics by allowing control over the basic properties of light (phase, amplitude and polarization) with subwavelength spatial resolution. One particular device that has attracted a lot of attention is the planar lens (also referred to herein as metalens). Due to their planar configuration, metalenses have potential applications in several multidisciplinary areas including imaging, spectroscopy, lithography, and laser fabrication.
High numerical aperture (NA) and high efficiency lenses have been demonstrated in both near infrared and visible spectrum. It has also been shown that multifunctional metalenses can be achieved without adding design and fabrication complexity. Multi-order diffractive lenses and multi-wavelength achromatic metalenses have been reported to compensate for chromatic dispersion at several discrete wavelengths, but achieving achromatic focusing over a significant bandwidth has proven challenging for Fresnel lenses and metalenses. Metalenses are used in a wide variety of applications, in which either an illumination source (e.g. LED imaging) or a signal (e.g. fluorescence and photoluminescence signals) has a substantial bandwidth. Broadband achromatic cylindrical lens in the visible spectrum also exist. However, they have a low numerical aperture (NA=0.013), specify three dimensional fabrication (grayscale lithography), and restrict the constituent materials to resists.